This invention relates generally to work vehicles having implements movable with respect to the vehicle by hydraulic cylinders. More particularly, it relates to dual hydraulic systems for providing additional hydraulic fluid to selected ones of the hydraulic cylinders.
Work vehicles, such as tractors, skid-steer loaders, backhoes, road graders, tele-handlers, and other similar vehicles, typically include a vehicle that travels over the ground and one or more implements that are attached to the vehicle and operate by the movement of hydraulic cylinders. The vehicles typically have a hydraulic power supply that provides a source of hydraulic fluid under pressure to the hydraulic cylinders, thereby moving the implements with respect to the vehicle. In addition, they typically have several hydraulic directional control valves that are controlled by the operator to conduct the hydraulic fluid flow to and from the hydraulic cylinders, thereby permitting the operator to control the implements.
Most of these vehicles typically have a single hydraulic power source that provided hydraulic fluid under pressure to all of the hydraulic cylinders. In many such vehicles, the hydraulic valves that are coupled between the hydraulic fluid source and the cylinders themselves are located in a single unitary bank of valves commonly called a xe2x80x9cvalve manifoldxe2x80x9d. With a common pressure source providing fluid under pressure to each of these valves, the various linkages and hydraulic cylinders must all be designed to handle the pressure provided by the hydraulic fluid source. Thus, for example, if the hydraulic fluid source provides fluid at 900 psi, and is provided to all of the valves, the linkages, hydraulic conduits, and hydraulic cylinders on the implement must all be designed to handle this high pressure.
This is not always practical, however. For example, on most such work vehicles, there are occasions in which one linkage or hydraulic cylinder does not need this great a hydraulic pressure. One example of this is the work vehicle commonly called a xe2x80x9cloader backhoexe2x80x9d. The backhoe implement or attachment on a loader backhoe is comprised of several jointed arms and a bucket or other work tool located at the far end thereof. It has been found that one of the hydraulic cylinders on the backhoe implement, a cylinder called the xe2x80x9cboom lift cylinderxe2x80x9d, would benefit if it could be connected to a source of hydraulic fluid at a pressure greater than that provided to the other hydraulic cylinders in the vehicle. This is because the boom cylinder is located near the base of the backhoe boom and lifts the entire boom with respect to the vehicle. Given the great overhanging bulk of the backhoe boom, a higher pressure provided to the boom lift cylinder would permit the backhoe arm to be raised and lowered much more easily and rapidly. This higher pressure is not necessary for the other hydraulic cylinders in the backhoe implement since they do not operate under the same loads as the boom lift cylinder.
An early solution to this problem was simply to increase the system pressure of the hydraulic supply and thereby providing hydraulic fluid flows at a much higher pressure to all of the valves and all of the cylinders in the backhoe implement. This solution was not acceptable, since raising the system pressure provided to the valve manifold and the hydraulic cylinders on the implement to which it is couple caused premature wear and failure of many of the structural components that did not need this additional pressure or forces to operate.
A second solution, shown in FIG. 1, was to provide two separate sources of hydraulic fluid under pressure: a first circuit operating at 2000 psi, and a second source of hydraulic fluid under pressure that operated at a higher pressure, such as 3000 psi. The operator was provided with a switch that permitted him to turn the second high-pressure supply on and off.
This arrangement was also unsatisfactory. Even though the operator could engage the second high pressure supply at will, there was still premature wear and damage to the other valves in the manifold, as well as the hydraulic cylinders and structural components in the backhoe implement to which the valves were coupled. All valves and all cylinders experienced the increased pressure, even though the increased pressure was not necessary for their operation even under high load operating conditions. Indeed, it is desirable to provide a higher pressure only to one or two of the hydraulic cylinders that move the implement, and for those implements only use the high-pressure hydraulic fluid occasionally for particular operations that require the additional pressure.
What is needed, therefore, is a system that permits high pressure to be applied only to particular hydraulic cylinders used to move an implement with respect to a work vehicle while applying a lower pressure to other similar cylinders. It would also be beneficial to provide a system in which single hydraulic pressure supply at low pressure could be applied to several hydraulic cylinders in a work vehicle and a second source of hydraulic fluid under pressure could be selectively applied to a subset of those cylinders. It would also be beneficial to provide a system in which a low pressure source of hydraulic fluid could be applied to a valve manifold and the plurality of valves in the manifold and a second higher pressure source of hydraulic fluid could be applied to a subset of those valves in the manifold. It is an object of this invention to provide these advantages.
In accordance with a first embodiment of the invention, a hydraulic system for operating an implement coupled to a work vehicle is provided, the system including a first hydraulic fluid source configured to generate a flow of hydraulic fluid at a first pressure, a second hydraulic fluid source configured to generate a flow of hydraulic fluid at a second pressure higher than the first pressure, a hydraulic fluid reservoir, a plurality of hydraulic cylinders coupled to the implement to move the implement, and a valve manifold including a plurality of hydraulic valves configured to control the flow of fluid to the plurality of hydraulic cylinders, the manifold defining a pressurized hydraulic fluid supply conduit, a low pressure hydraulic fluid return conduit, and an open center conduit that is closed by actuation of the plurality of hydraulic valves, wherein the first fluid source is coupled to the manifold to provide hydraulic fluid under pressure to each of the plurality of valves and through those valves to each of the plurality of hydraulic cylinders, and wherein the second fluid source is directly coupled to at least one of the plurality of cylinders to supply fluid to the at least one cylinder. The second fluid source may be coupled to the manifold and may be in fluid communication with the open center conduit. The system may include a boost valve disposed to control fluid flow between the second fluid source and the manifold, and to control flow between the second fluid source and the at least one cylinder. The plurality of valves may include at least one operator actuable control valve for controlling flow in both directions to at least one cylinder. The boost valve may be configured to conduct fluid from the second fluid source to the at least one cylinder when at least one valve is opened by the operator. The work vehicle may be a backhoe and at least one cylinder may be a boom lift cylinder.
In accordance with a second embodiment of the invention, a hydraulic system for a backhoe is provided, including a first hydraulic fluid source configured to generate a flow of hydraulic fluid at a first pressure, a second hydraulic fluid source configured to generate a flow of hydraulic fluid at a second pressure higher than the first pressure, a hydraulic fluid reservoir, a boom swing cylinder, a boom lift cylinder, a dipper cylinder, and a bucket cylinder, a valve array including a boom swing cylinder valve, a boom lift cylinder valve, a dipper cylinder valve, and a bucket cylinder valve, in fluid communication with the boom swing cylinder, the boom lift cylinder, the dipper cylinder, and the bucket cylinder, wherein each of the valves in the valve array are in fluid communication with the first fluid source and are disposed to regulate fluid from the first fluid source to their respective cylinders, and wherein the second fluid source is directly coupled to the boom lift cylinder, and a pressure boost valve in fluid communication with the boom lift cylinder and the second fluid source that is configured to provide fluid flow from the second fluid source to the boom lift cylinder when the boom lift cylinder valve is opened. The boom lift cylinder may have an extend port and a retract port configured to extend and retract the boom lift cylinder when fluid is introduced into each respective port. The boom lift cylinder valve may have an extend port and a retract port that are in fluid communication with the extend and retract ports of the boom lift cylinder, and may also have a reservoir port and a supply port that are in fluid communication with the reservoir and the first fluid source, respectively. The second fluid source may be in fluid communication with one of the extend and retract ports of the boom lift cylinder, and with the port of the boom lift cylinder valve that is in fluid communication with the one port.